Proliferating cell nuclear antigen (PCNA) plays an important role in the process of DNA replication, repair, chromosomal recombination, cell cycle check-point control and other cellular proliferative activities. In conjunction with an adaptor protein, replication factor C (RFC), PCNA forms a moving clamp that is the docking point for DNA polymerases delta and epsilon. Different isoforms of proliferating cell nuclear antigen (PCNA) that display both acidic and basic isoelectric points (pI) have been demonstrated. Analysis of PCNA by two-dimensional polyacrylamide gel electrophoresis (2D PAGE) from both malignant and non-malignant breast cells (referred to as non-malignant PCNA or nmPCNA) and tissues revealed the presence of an acidic form of PCNA only in malignant cells (referred to as the cancer-specific PCNA or csPCNA or caPCNA). This difference in isoelectric points between these two forms of PCNA appears to result from an alteration in the ability of the malignant cells to post-translationally modify the PCNA polypeptide and is not due to a genetic change within the PCNA gene.
Structural work examining the structure of the PCNA polypeptide to define the structural differences between the caPCNA and non-malignant cell isoform of PCNA revealed a region of the caPCNA protein that is uniquely exposed only in the cancer cell. An antibody was developed to a region of the cancer specific isoform of PCNA that is highly selective for the PCNA isoform expressed exclusively in cancer cells.
Proliferating cell nuclear antigen (PCNA) is a 29 kDa nuclear protein and its expression in cells during the S and G2 phases of the cell cycle makes the protein a good cell proliferation marker. It has also been shown to partner in many of the molecular pathways responsible for the life and death of the cell. Its periodic appearance in S phase nuclei suggested an involvement in DNA replication. PCNA was later identified as a DNA polymerase accessory factor in mammalian cells and an essential factor for SV40 DNA replication in vitro. In addition to functioning as a DNA sliding clamp protein and a DNA polymerase accessory factor in mammalian cells, PCNA interacts with a number of other proteins involved in transcription, cell cycle checkpoints, chromatin remodeling, recombination, apoptosis, and other forms of DNA repair. Besides being diverse in action, PCNA's many binding partners are linked by their contributions to the precise inheritance of cellular functions by each new generation of cells. PCNA may act as a master molecule that coordinates chromosome processing.
PCNA is also known to interact with other factors like FEN-1, DNA ligase, and DNA methyl transferase. Additionally, PCNA was also shown to be an essential player in multiple DNA repair pathways. Interactions with proteins like the mismatch recognition protein, Msh2, and the nucleotide excision repair endonuclease, XPG, have implicated PCNA in processes distinct from DNA synthesis. Interactions with multiple partners generally rely on mechanisms that enable PCNA to selectively interact in an ordered and energetically favorable way.
The use of short synthetic peptides for the generation of polyclonal and monoclonal antibodies has been successful. Peptides are known to serve as chemo-attractants, potent neurological and respiratory toxins, and hormones. The peptides have also been used as affinity targets and probes for biochemical studies, and have provided a basis for understanding the characteristics and specific nature of discrete protein-protein interactions. In addition, peptide hormones exert potent physiological effects, and in some cases the active hormone is either a peptide that is contained within a larger protein or is processed and released from a precursor protein prior to exerting its physiological effect.
Peptides have been used to disrupt protein-protein interactions, by acting as highly specific competitors of these interactions. Biochemical studies employing peptide reagents advanced the use of peptides as therapeutic drugs capable of disrupting cell functions that require protein-protein interactions. Thus, specific cellular processes such as apoptosis and cell cycle progression, which are dependent upon discrete protein-protein interactions, can be inhibited if these protein-protein interactions are selectively disrupted. The replication of genomic DNA being dependent on protein-protein interactions is also susceptible to peptide-induced inhibition of these protein interactions.
In vivo DNA synthesis is a highly regulated process that depends on a myriad of biochemical reactions mediated by a complex series of protein-protein interactions. Cell division is dependent on the DNA synthetic process, and cancer cell growth is substantially sensitive to any agent that disrupts the regulation and/or the activity of the DNA synthetic machinery responsible for copying the cancer cell's genomic DNA. In addition, it was demonstrated that one signature of cancer, for example, breast cancer, is the induction of genomic instability, as transformed cells develop a highly aggressive metastatic phenotype. Genomic instability arises through a series of changes in the cellular DNA synthetic machinery that alters the fidelity with which DNA is synthesized.
Studies utilizing the carboxyl terminal 26 amino acids from the p21cip protein, (which is known to interact with the PCNA protein), demonstrated the ability of this peptide to disrupt the cellular proliferative process. This peptide fragment of p21 potentially disrupts one or more cellular processes utilizing PCNA and presumably interferes with protein-protein interactions that participate in the DNA synthetic process as well as the regulation of other cell cycle check-point controls and the induction of apoptosis.
Studies utilizing this peptide fragment of p21 have demonstrated the ability of the p21 peptide to activate a non-caspase associated apoptotic pathway. Similarly, studies involving a 39 amino acid peptide fragment of the p21 protein partially inhibited DNA replication in vivo, and suggest that this peptide fragment of p21 can stabilize the PCNA-p21 protein interaction leading to the decrease in DNA synthetic activity within the cell.
In addition, computational chemical methods are being used to model specific regions of the PCNA molecule that may interact with other cellular proteins involved in cell cycle check point control and DNA synthesis. Regions of the cyclin-CDK complex may serve as templates to identify target sites for disrupting key cell cycle check-point control points that are essential for cell proliferation.
Use of synthetic peptides to inhibit cell proliferation and the process of selectively targeting cancer specific PCNA protein to mediate the inhibition of cell proliferation is needed to treat cancer. Peptidomimetic drugs that interact with an antigenic site or target site on caPCNA to disrupt specific protein-caPCNA interactions that are unique to the cancer cell are desired. Peptides derived from caPCNA specific epitopes, described herein, significantly augment the cytotoxic effects of standard chemotherapeutic regimens and consequently kill cancer cells in a highly selective manner.
Germ-line mutations in BRCA1 or BRCA2 alleles are associated with a high risk of the development of a number of cancers, including breast, ovarian, and prostate cancer. Cells lacking these or other important DNA repair proteins have deficiencies in the repair of DNA double stranded breaks by homologous recombination. For example, loss of BRCA1 function often leads to aggressive tumors, and the tumors are often resistant to chemotherapeutic DNA damaging agents. Thus, novel therapeutics, such as caPCNA peptides, that exploit the DNA repair defects in cancers harboring mutations in homologous recombination pathways, are advantageous to standard chemotherapeutics used alone.